In recent years, with a remarkable spread of mobile phones, cordless phones, patch antennas for RFID and the like, lens antennas such as a radio telescope, a millimeter-wave radar, and the like and with significant progress of satellite communication apparatuses, a communication signal is demanded to have a high frequency use, and the communication apparatus is demanded to be more compact. As the dielectric constant of a material for an antenna incorporated inside the communication apparatus becomes higher, the communication signal is allowed to have a higher frequency, and the communication apparatus is allowed to be more compact. As the dielectric dissipation factor of a material for an antenna becomes lower, it is possible to decrease power consumption at the time of communication. The dielectric constant and the dielectric dissipation factor are parameters indicating the extent of a polarity inside a dielectric and the loss of an energy generated by polarization inside the dielectric or by application of conductivity thereto respectively.
As an elastomer dielectric material for the antenna, an ethylene propylene rubber material having not less than 15 in its dielectric constant is known (see patent document 1). Existing dielectric materials contain dielectric ceramic powder at a high ratio to allow the antenna to be compact. In recent years, with an increase of use of RFID in a band of 950 MHz, dielectric materials having 4 to 10 in the dielectric constant thereof and not more than 0.02 in the dielectric dissipation factor thereof are demanded. When the dielectric constant is larger than 10, a small antenna is produced. Consequently a small gain is obtained, which is unpreferable. When the gain is small, the distance in which communication can be accomplished becomes short.
As the dielectric materials having 4 to 10 in the dielectric constant thereof and not more than 0.02 in the dielectric dissipation factor thereof, those made of ceramic and resin are commercially available, but are inferior to the elastomer dielectric material in the shock resistance, flexibility, and processability thereof, which is unpreferable.
As compositions, composing elastomer composite materials, which have 4 to 10 in the dielectric constant thereof and not more than 0.02 in the dielectric dissipation factor thereof, those containing an ethylene elastomer or a styrene elastomer and fibrous dielectric ceramics added thereto are known (see patent document 2).
But the data of each of the dielectric properties described in the patent document 2 are values measured at 1 MHz or 3 GHz. The dielectric properties are not measured at 950 MHz. No description is made on the hardness of the composition, and sufficient optimization is not accomplished in consideration of the shock resistance, flexibility, and processability thereof.
In producing a patch antenna to be used at 950 MHz from a dielectric material having a dielectric constant of 4 to 10, a dimension not less than 100 mm is necessary. But it is expensive to form a sheet from ceramics or resin when the sheet has a dimension not less than 100 mm.
Patent document 1: Japanese Patent Application Laid-Open No. 2005-89686
Patent document 2: Japanese Patent Application Laid-Open No. 9-31244